How can I smoothly transition between two UV maps?
I'm with Ray on this. It sounds like the wrong solution for the problem at hand.

Anyway, you can't really "change between" UV maps, but you can transition between them at shader level.

Since UVs are vectors like RGB values, you can work around it by using a Color Mix node to "mix" between two UV layers.



UVWarp, or UV Project may be more adequate solutions